Many first-time pregnant women think they know about the physical changes that accompany pregnancy, almost all of which they anticipate happening somewhere in the vicinity of their belly button. They expect to feel nauseated, to want to eat a lot, and to go to the bathroom with nearly comic frequency. All of these make a certain kind of sense, since they have something to do with the area where the baby is growing.

During my first trimester, however, I found myself out of breath after only a short walk. I didn't think it had anything to do with my pregnancy; after all, I wasn't even showing. How could the baby put pressure on my lungs? But when I looked it up in a reference book, I found it: breathlessness, a result of pregnancy hormones, common in the first trimester.

"I spend about 80 percent of my time with patients explaining that what they're experiencing is normal," says Thomas Moore, M.D., chairman of the department of reproductive medicine at the University of California, San Diego. And many of the often bizarre changes can be attributed to that familiar culprit: hormones. Some of the same ones that made you a little cranky and uncomfortable, or gave you blemishes in high school—primarily estrogen and progesterone—come back into play. Between the time of conception and your seven-month mark, they'll triple in quantity, then remain at that level until you deliver.

But just as every teenager reacts differently to puberty, every mom-to-be responds differently to this hormonal surge. For some, pregnancy is a time of glowing skin and high energy; for others, it's a return to concealers and an introduction to nearly constant nausea. Some are highly sexual, others lose their desire; some are ravenous, others can barely force anything down.

Doctors don't know why there's such a wide range of reactions. What they do know is that in all pregnant women, the hormones work toward the same goals. To start with, they instruct the kidneys to retain salt and water in order to build blood volume. By her seventh month, a pregnant woman will have 40 percent more—enough to circulate through the placenta. To help cope with this increase, which would normally cause high blood pressure, the hormones also lower blood pressure, which allows the heart to do more without added stress. As part of this process, progesterone relaxes the involuntary muscles of the intestines, bladder, stomach, and kidneys. Connective tissue and ligaments also loosen, to help the baby pass through the pelvis more easily.

But while such side effects as nausea, backaches, and leg cramps are part and parcel of creating a healthy infant, they aren't much fun for Mom. "The baby is taking you over," says Dr. Moore, "and he isn't doing it for your well-being."

Thankfully, most symptoms aren't threatening. But since a few can be a sign of more serious problems, bring any to the attention of your obstetrician. For benign conditions, the main thing she'll offer—aside from home remedies and certain over-the-counter drugs—is reassurance. The good news is, many women find maladies ease as they enter the next trimester (even if new ones arise). From head to toe, changes you might not expect:

Head

With increased blood circulation, your entire body will be a little swollen. About 10 weeks into your pregnancy, you may find contacts don't fit as well over enlarged eyes, and your vision may not seem as sharp—both temporary conditions. If you experience a sudden onset of blurry vision, however, check with your obstetrician immediately. It may signal dangerously high blood pressure, known as preeclampsia.

Engorged nasal tissue can bring on nosebleeds and stuffiness. But allergy sufferers may find relief: Strong immune reactions trigger allergies, and progesterone and higher levels of the hormone cortisone suppress the immune system.

Many women also experience headaches in the first trimester, which can be blamed on low blood sugar—the result of your changing metabolism—or reduced blood flow to the brain when you stand or sit up quickly. "Your uterus has first dibs on your blood supply," says William Sears, M.D., a Parenting contributing editor and coauthor of "The Pregnancy Book." Dizziness occurs for the same reason.

Extra fluids can also create excess saliva, as well as swollen and bleeding gums. And some women develop tiny growths on their gums. "They're probably a result of the same hormones that are helping your body grow your baby, and will disappear after delivery," says Dr. Sears.

Finally, your cheeks may glow (from oil-gland secretions, plus more blood to the skin) or sport brown or yellow patches (increased estrogen and progesterone result in more pigment, especially noticeable when exposed to the sun). This discoloration is similar to what some women experience when taking the Pill.

Hands, Wrists, and Arms

Side effects: Carpal tunnel syndrome, red palms

In the third trimester, because of fluid retention, the nerves snaking through tiny passageways are likely to be pinched by swollen tissue, which can lead to, or worsen, carpel tunnel. Occasionally, the shooting pain, numbness, and tingling can migrate to the forearm and shoulder.

As veins enlarge to accommodate greater blood volume, and capillaries branch out to handle the increase, the skin all over your body enjoys greater blood flow. Consequently, some women's palms (or soles of the feet) become reddened after about 12 weeks.

Chest

Pregnant women actually breathe easier; they just don't always realize it. Progesterone reprograms the brain so that you'll inhale 30 to 40 percent deeper to supply all that extra blood with oxygen. Some women even add inches to their rib cage as a result of increased lung size. "Your bones change in proportion to the tissue you have to support. After delivery, they'll probably return to normal," says Dr. Sears.

On one hand, this improved breathing capacity, combined with more cortisone, means about a third of asthmatics can—with their doctor's permission—go off their medication. (In another third, the asthma gets worse; it stays the same for everyone else.) On the other hand, many pregnant women continue to feel breathless even though they're getting plenty of air, partly because the baby is transferring more carbon dioxide to you. In the last trimester, breathlessness occurs because the growing uterus limits lung space.

Heartburn (or gastroesophageal reflux) is caused by the relaxation of intestinal muscles in the first trimester. Digestion slows, letting food sit longer and creating more acid in your stomach. At the same time, this effect loosens control of the esophagus, which separates the stomach and throat. The contents of the stomach are then able to move backward into the esophagus, or even higher in the throat.

The other chest-related changes—larger, more sensitive breasts, and darker nipples (from extra pigment)—are more predictable, but may come with surprises of their own. Many women have breast tissue all across their chest and in their armpits, and hormones can cause it to swell into lumps.

Hormones also stimulate mid-pregnancy growths on the chest (and elsewhere), such as tiny, harmless polyps of skin, known as skin tags, and hair where there wasn't any. When hormone levels drop after birth, these usually disappear.

Back

Side effect: Backache

Back pain can become a problem for pregnant women long before an enlarged belly causes the changes in posture that pull and strain muscles. Why? As early as eight weeks after conception, connective tissue in the pelvis softens, and the sacroiliac joint in the lower back loosens. In the third trimester, sciatic pain can develop if the baby parks himself on the nerve.

Abdomen

Side effects: Frequent urination, bladder infections, itchiness

You knew you would go to the bathroom a lot, but you may not have known that you would go so little, so frequently. That's because the bladder never completely empties during pregnancy; it's 30 to 50 percent full at any given moment as a result of progesterone's relaxing effect on the muscles. At the same time, the kidneys work about 40 percent harder to remove waste from both you and the baby. Aside from the uncomfortable sensation of never emptying your bladder, this phenomenon also increases the risk of urinary infection—and may make it harder to detect when you have one. If going to the bathroom becomes painful at any time, contact your obstetrician.

You may experience itchiness during the third trimester, especially on your abdomen. It's typically the annoying but harmless result of hormones and stretching skin. If it becomes intense or prolonged, though, tell your doctor; it could indicate a liver problem known as choletasis.

Legs and Feet

Side effects: Cramps, varicose veins, swelling

Cramps in your feet and calves are normal in the second and third trimesters, and may be caused by either fatigue or the uterus putting pressure on the nerves in your legs.

Varicose veins and swollen feet are two other ailments that usually occur once your baby has made her physical presence known. At that point, the weight and position of the uterus impairs blood flow, enlarging already swollen veins in the legs, rectum (hemorrhoids), and vulva. As they become more visible, they'll sometimes create discomfort. Your feet are likely to swell as part of the fluid-retention process, and because of your increasing weight.

An End in Sight

As you observe the changes taking place in your body, take comfort in knowing that what pregnancy gives, childbirth most often takes away. Around three hours after delivery, some hormone levels return to normal, says Dr. Moore, often quickly eliminating many bothersome side effects that have lasted for months.

In the days following my daughter's birth, I remember gleefully stretching my feet and pointing my toes (no cramps), walking up the stairs (without losing my breath), and ceremoniously dropping my bottles of antacid in the trash one by one. Ultimately, I was left with the one effect of pregnancy that really lasted: Her name is Anna.